Safety devices are known, as disclosed in the applicant's earlier GB 2335720, for indicating relative rotational displacement between a wheel nut and a threaded stud to which it is engaged so as to indicate whether or not a wheel nut has become loose, in which case it can be tightened. The known devices comprise a body having a bore formed with equally spaced grooves enabling the device to be releasably secured to the wheel nut in any one of a plurality of positions, the grooves enabling the device to be secured to the nut by engagement of the corners or apices of the nut. The particular safety devices disclosed in above noted specification and commercially available under the name “Dustite” (registered trademark) have a closure at one end of the bore which is formed integrally with the body so as to serve as a dust cap for the wheel nut. However, as disclosed in another of the applicant's prior specifications, GB 2325505, such an end closure may be provided as a removable cap. Further, of course, these rotation indicator devices have indicating means, typically in the form of a pointer, which is either formed integrally with the body or separately and releasably connected thereto, or integrally with any removable cap, and which typically projects laterally from the body. However, such indicating means can be provided as a marker or formation (for example an arrow) on the body which does not project outwardly, and this may be useful in situations other than vehicle wheels, where there is limited space adjacent a nut or bolt head to which the device is to be applied.
Conventional nuts and bolt heads have a polygonal cross-section or perimeter, most usually hexagonal, but square is also a shape sometimes used, onto which the grooves of the known safety devices are able to releasably engage. The hexagonal or square shape also serves the primary function of enabling engagement of a fastening tool, such as a spanner or wrench. In some situations, however, alternative screw threaded fasteners are used, typically those known as Allen bolts, which have a head with a cylindrical perimeter (circular cross-section) and a central recess in the head for engagement by a fastening or unfastening tool, typically known as an Allen key. These types of fasteners may be used on flanges of pipework where it may be desirable to use a rotation indicator to show if any are working loose. The applicant's known devices are not suitable in these circumstances because the lack of corners on the Allen bolt head perimeter prevent secure, non-rotatable attachment of the known devices when fitted over said bolt heads. This is the case even when the perimeter of the Allen bolt head is formed with splines, namely a series of narrow longitudinal ridges formed by intermediate narrow grooves.
An object of the present invention is to provide a safety device, namely a rotation indicating device, comparable to known devices, but which can be used effectively on an Allen bolt.